Trench isolation for semiconductor devices

ABSTRACT

In etching trench isolation structures, a pad oxide or sacrificial oxide may be formed with substantially the same (or higher) etch rate as the trench filler. Because the etch rate in the trench area is substantially similar to (or less than) the etch rate in the non-trench area, similar amounts of material are removed in both the trench area and non-trench area in a subsequent etching process. Consequently, formation of notches and grooves in the semiconductor structure is minimized. A sacrificial oxide layer may be made by depositing a layer of a suitable material on the surface of a semiconductor structure. By depositing a sacrificial oxide layer instead of thermally growing a sacrificial oxide layer, grooves and the notches in the trench areas are filled by the deposited material.

The invention relates to fabricating semiconductor devices andparticularly to trench isolation structures.

BACKGROUND

Trenches formed in the substrate and filled with oxide are widely usedas isolation structures. Shallow trench structures can be used toisolate adjacent electronic devices, such as transistors. Deep trenchisolation structures can be used to isolate N-wells and P-wells in thesubstrate. The trench isolation technology has proved to be important infabricating large scale integrated circuits that require high density ofelectronic devices.

Trench isolation structures may be formed in a semiconductor substrateby depositing silicon dioxide in the isolation trench. A silicon dioxidelayer is thermally grown over the surface of a silicon substrate. Next,a silicon nitride layer is deposited by chemical vapor deposition. Thelayers are then patterned to define the width of a trench to be etchedin the silicon substrate. The exposed surface, in this case siliconnitride, is then removed using plasma or chemical etching. Similarly,silicon dioxide is removed by chemical or plasma etching. Followingthis, the exposed surface of the substrate, in this case silicon, isfurther removed to create a trench.

After a desired trench depth is obtained, the etch process stops. A thinsilicon oxide layer is thermally grown in the trench area. This trenchoxide layer is formed to passivate the interior walls and bottom of thetrench which may have been slightly damaged during the trench etch.

Upon completion of the trench oxide layer, deposited silicon dioxide isused to fill the trench. After the deposition of silicon dioxide, thesubstrate is planarized. During planarization, silicon dioxide isremoved completely over the silicon nitride hard mask. The siliconnitride layer is subsequently etched away. In this structure, depositedsilicon dioxide covers the trench area, whereas thermally grown silicondioxide, called pad oxide, covers the non-trench area.

In a subsequent step of etching away the pad oxide layer, grooves andnotches indicated as "A" may be created in the trench oxide surface area60a, which is illustrated in FIG. 2.

A sacrificial oxide layer may be thermally grown onto a substrate aftertrench formation. After implantation, the sacrificial oxide layertypically is removed by chemical etching.

Because thermally grown silicon dioxide can only form in the area wherethere is silicon, this grown silicon dioxide layer covers only thenon-trench area. As a result, the trench area is not covered byadditional silicon dioxide. When the sacrificial oxide layer 80a issubsequently etched away, some of the trench fill oxide 60a will beetched away, as indicated at "A" in FIG. 3.

The formation of grooves and notches of this sort may adversely impactthe operability and reliability of an integrated circuit.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method offabricating a semiconductor device includes depositing a first materialon a surface, the material having a first etch rate. A trench is formedthrough the first material and into the surface. A trench fillermaterial is deposited in the trench. The deposited trench fillermaterial has an etch rate that is substantially similar to or less thanthe first etch rate.

In accordance with another aspect of the present invention, a method ofproviding a sacrificial layer on a semiconductor structure having atrench structure filled with a trench filler material includesdepositing a first material over the trench filler material and thesurface.

The layer is annealed.

In accordance with yet another aspect of the present invention, asemiconductor structure includes a support and a first materialdeposited on the support. A trench is formed through the first materialand into the support. A trench filler material deposited in the trench.The deposited trench filler material has an etch rate that issubstantially similar to or less than the etch rate of the firstmaterial.

In accordance with still another aspect of the present invention, asemiconductor structure having a trench includes a trench fillermaterial that fills the trench. At least a portion of a second materialis deposited on the trench filler material. The second material isannealed.

In accordance with but another aspect of the present invention, a methodof forming trench isolation structures in semiconductor devices includesforming a layer on a semiconductor member and forming a trench throughthe layer and into the member. The trench is filled with a trenchfilling material. The trench filling material is etched and the layer isremoved without forming grooves in the remaining trench fillingmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1G are enlarged cross-sectional views of a semiconductorsubstrate at several steps in fabricating a trench isolation structureaccording to one embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a semiconductor trenchisolation structure according to a prior art method.

FIG. 3 is an enlarged cross-sectional view of a sacrificial oxide layeron a semiconductor substrate with a trench isolation structure accordingto a prior art method.

FIGS. 4A-4B are enlarged cross-sectional views of a semiconductorsubstrate showing the process for making a sacrificial oxide layer on asemiconductor substrate with a trench isolation structure according toone embodiment of the present invention.

FIG. 5 is a flow diagram of a method of fabricating a device on asemiconductor substrate according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, numerous specific details are set forthsuch as specific thickness, materials, processing sequence, depositionmethods, temperature, etch rates, etc. It will be evident to one ofordinary skill in the art that the present invention may be practicedwithout these specific details. Well known processing steps, equipment,etc., are not described in particular detail.

Initially, a pad oxide layer 20 is deposited on the surface of asemiconductor structure 10 as illustrated at step 100 in FIGS. 1A and 5.The semiconductor structure 10 may take various geometric shapes; it maybe made of one or more semiconducting materials. The semiconductorstructure includes not only a structure formed by a semiconductor, butalso a structure formed by a semiconductor and a non-semiconductingmaterial, such as a conductor or an insulator.

The pad oxide layer 20 provides a "pad" or "buffer" to cushion thetransition of stress between substrate 10 and subsequent layers. The padoxide 20 can be formed by depositing SiO₂, Ta₂ O₅, Al₂ O₃. Depositionmethods include low pressure chemical deposition (LPCVD), plasmaenhanced vapor deposition (PECVD), and thermal decomposition. Variousstarting materials known in the art may be used in these depositionmethods. For example, to deposit SiO₂, tetraethylorthosilicate, SiH₄+O₂, and SiH₄ +N₂ O, SiCl₂ H₂ +N₂ O may be used. In one embodiment, SiO₂is deposited at 660° C. in a LPCVD reactor by decomposingtetraethylorthosilicate, Si (OC₂ H₅)₄. This compound, also known asTEOS, is vaporized from a liquid source. The overall reaction is:

Si (OC₂ H₅)₄ →SiO₂ +by products, where the byproducts include a complexmixture of organic and organosilicon compounds. During the deposition,the pressure of the reaction chamber may be maintained at approximately1 Torr. The thickness of the SiO₂ pad oxide layer 20 can be about 60 Å,for example.

There are some important differences in physical properties betweenthermally grown silicon dioxide and deposited silicon dioxide. Thermallygrown silicon dioxide consists of stoichiometric SiO₂, while depositedsilicon dioxide contains SiO₂ and other impurities such as carbon,hydrogen, and water. Another difference is in their etch rates.Thermally grown silicon dioxide has an etch rate of 25 Å per minute inan HF solution (H₂ O:HF:100:1). In contrast, the etch rate of adeposited silicon dioxide is generally equal to or greater than 30 to 50Å per minute. In the case of plasma deposited silicon dioxide at a lowtemperature, the etch rate in such a solution may be as high as 400 Åper minute.

At step 150 in FIG. 5, silicon nitride layer 30 is deposited, as shownin FIG. 1A. Methods for depositing silicon nitride are generally wellknown in the art. For example, silicon nitride can be chemicallydeposited by reacting silane and ammonia at the atmospheric pressure ata temperature between 700 and 900° C. or by reacting dichlorosilane andammonia at a reduced pressure at a temperature between and 700 and 800°C. Typically, the pressure of the reaction chamber is maintained around250 millitorr. The silicon nitride layer can have a thickness of 2000 Å,for example.

Once the pad oxide layer 20 and silicon nitride layer 30 have beendeposited, a trench 40, as illustrated in FIG. 1B, is formed in thestructure 10 at step 200. One method of forming the trench is to firstcover the layers 20 and 30 on the structure 10 with a photoresist layer(not shown), which is then masked, exposed, and developed to defineopenings in the photoresist layer where trenches are formed in thesubstrate. Then, the silicon nitride and pad oxide are etched in thedefined trench areas. Etching may be achieved by using a plasma etchmethod with a chemistry including HBr and NF₃ or SiF₄. Other etchmethods known in the art may also be used.

After the trench has been formed in the substrate, a trench oxide layer50 (FIG. 1C) is formed at step 250. The oxide layer may be thermallygrown or deposited. For example, a 150 Å SiO₂ layer can be thermallygrown in an oxygen ambient.

At step 300, a trench filler material 60 (FIG. 1D) is deposited in thetrench to fill the trench. Advantageously, the trench filler materialshould have an etch rate that is substantially similar to or less thanthat of the pad oxide deposited at step 100. A trench filler materialhas a substantially similar etch rate to that of the pad oxide if theratio between the two etch rates is less than 1.2. One way to achievesuch a result is to use the same deposition process at step 300 as theone used at step 100. In one embodiment, TEOS is used to deposit silicondioxide in both step 100 and step 300 by LPCVD. In addition to TEOS,silane or dichlorosilane may also be used to deposit silicon dioxide. Inanother embodiment, plasma enhanced chemical vapor deposition is used todeposit silicon dioxide. Plasma may be initiated by radio frequency,microwave or by a thermal method.

Although silicon dioxide is used in one embodiment, it is to beunderstood that other materials may be used as the trench fillermaterial and the pad oxide, such as Al₂ O₃. Advantageously, the trenchfiller material and the pad oxide layer material are electricallyinsulating, act as a stress transition layer if deposited between layer30 and substrate 10 and may be etched away without substantialdifficulty.

After the trench 40 has been filled by the trench filler material 60,the excess trench filler material is planarized at step 350.Planarization methods are well known in the art. If the trench fillermaterial 60 is silicon dioxide deposited using TEOS, a chemicalmechanical polish (CMP) can be used to remove the excess silicondioxide. Here, a polish slurry with a chemistry including KOH and silicais used while the excess trench filler material is being polished by apolishing pad.

After planarizing the excess trench filler material, the silicon nitridelayer 30 is removed at step 400 of FIG. 5 by such methods as wetchemical etch, reactive ion etching, reactive ion beam etching, andmicrowave plasma etching. For example, CF₄ +O₂ may be used in plasmaetching of silicon nitride.

After the silicon nitride layer 30 has been removed, as shown in FIG.1E, the trench area is covered by trench filler material 60, whereas thenon-trench area is covered by pad oxide layer 20. Because the trenchfiller material 60 and pad oxide layer 20 may have substantially similaretch rates, they are removed at a substantially similar rate at step450. Any known etch method may be used so long as the etch rate in thetrench area is substantially similar to (or less than) the etch rate inthe non-trench area. In one embodiment where both the trench fillermaterial and the pad oxide include silicon dioxide deposited from TEOSby LPCVD, an HF solution is used as the etchant. The ratio of HF to H₂ Ois in the range of 10:1 to 100:1. After the pad oxide layer 20 isremoved at step 450, a semiconductor device with a trench filled by atrench filler material is obtained (FIG. 1F).

Because of the substantially similar etch rates of the pad oxide 20 andtrench filler oxide 60, the pad oxide 20 can be etched while stillmaintaining the trench area substantially free of notches or voids. Thepresence of notches or voids may be detrimental to the performance of anelectronic device. If polysilicon particles fall into these notches orvoids, shorts may occur between devices that the trench is supposed toisolate. Such a notch may cause transistor characteristics to benon-ideal. For example, the threshold voltage may be lower at a notch.

In one embodiment, a sacrificial oxide layer 70 (FIG. 1G) is depositedat step 500 (FIG. 5). The thickness of the sacrificial oxide layer maybe about 250 Å, for example. In comparison to thermally grown siliconoxide as a sacrificial oxide layer, deposited oxide can fill grooves andnotches in the trench area created during etching. This advantage mayfurther be enhanced by a thermal annealing step (step 550). In thisstep, the as-deposited oxide layer is subjected to a heat treatment at atemperature above about 900° C. to densify the deposited material. Otherknown thermal annealing methods can be used.

The annealing step may be done in an oxidizing environment to form athin grown oxide which is part of the layer 70. Alternatively, a thingrown oxide may be formed before the rest of the sacrificial oxide layer70 is formed by deposition.

After annealing, a threshold voltage adjustment implant may be performedat step 600. The implanted dopant may be boron or phosphorus, forexample. At step 650, the sacrificial oxide layer is removed by etching.At step 700, a gate oxide is grown, followed by depositing polysiliconor silicide onto the gate oxide. Transistors are then formed in theensuing steps using conventional techniques.

In FIG. 4A, a semiconductor substrate 10 with a trench isolationstructure made by a prior art method is shown. Such a structure hasnotches and grooves "A" in the trench area. In FIG. 4B, an oxide layer80 is deposited on the surface formed by the trench filler material andthe substrate. In one embodiment, silicon dioxide is deposited from TEOSby LPCVD. It is to be understood that the etch rate of the sacrificialoxide is not required to be similar to that of the trench fillermaterial. However, it is advantageous to select an oxide that has anetch rate substantially similar to the etch rate of the trench fillermaterial. Because silicon oxide is deposited rather than thermallygrown, the deposited oxide may fill the notches and grooves in thetrench area. In another embodiment, silicon dioxide is deposited from asilane and oxygen gas mixture. Any deposition method known in the art,such as LPCVD or PECVD, may be used. Advantageously, the as-depositedsacrificial oxide layer is annealed and densified at a temperature above900° C.

While the present invention has been disclosed with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of the present invention.

What is claimed is:
 1. A method of fabricating a semiconductor device,comprising:depositing a first material on a surface, the first materialhaving a first etch rate; forming a trench through the first materialand into the surface; depositing a trench filler material in the trench,the trench filler material having an etch rate that is less than 1.2times the first etch rate; forming a sacrificial layer of a secondmaterial on the trench filler material; and annealing the sacrificiallayer to form a thermally grown portion of said sacrificial layer. 2.The method of claim 1, including removing the first material.
 3. Themethod of claim 2, including depositing at least a portion of asacrificial layer of a second material on the trench filler material andthe surface.
 4. The method of claim 3, including implanting a dopantinto the substrate through the sacrificial layer and removing thesacrificial layer.
 5. The method of claim 3, including forming asacrificial layer by deposition and thermal growth.
 6. The method ofclaim 2, including depositing a sacrificial layer of a second materialon the trench filler material, the second material having an etch ratesubstantially similar to or less than the etch rate of the trench fillermaterial.
 7. The method of claim 6, including annealing the sacrificiallayer.
 8. The method of claim 1 wherein the first material includes anoxide.
 9. The method of claim 8 wherein the first material includessilicon dioxide deposited from tetraethylorthosilicate or a silane andoxygen system.
 10. The method of claim 8 wherein the first material isdeposited by chemical vapor deposition.
 11. The method of claim 7wherein the trench filler material includes an oxide.
 12. The method ofclaim 11 wherein the trench filler material is deposited by chemicalvapor deposition.
 13. The method of claim 8 wherein the first material,the second material and the trench filler material are silicon dioxide.14. A method of covering a semiconductor structure having a trenchstructure filled with a trench filler material, comprising:depositing alayer of a first material on the trench filler material and thestructure; growing an oxide layer after said layer of a first materialis deposited; and annealing the layer of a first material.
 15. Themethod of claim 14 wherein the etch rates of said trench filler materialand said first material are substantially similar.
 16. The method ofclaim 15 wherein both the first material and the trench filler materialinclude silicon dioxide.
 17. The method of claim 16 wherein the silicondioxide is deposited from tetraethylorthosilicate or a silane and oxygensystem.
 18. The method of claim 16 wherein the silicon dioxide isdeposited by chemical vapor deposition.
 19. The method of claim 14,including implanting a dopant into the substrate through the sacrificiallayer.
 20. The method of claim 14, wherein said oxide layer is grownduring said annealing step.